The present invention relates generally to echo cancellers for a digital communications system, and more particularly to an echo canceller capable of cancelling echoes having a long tail portion.
Two-wire subscriber lines employing an echo canceller are known in the art. The echo canceller of this type has a nonrecursive, or finite impulse response (FIR) filter. Because of the finite number of tap weights, the whole shape of an echo cannot completely be cancelled if it has a long tail portion. A great number of tap weights must be required for complete cancellation of echoes. It is generally known that such a long-tail echo occurs due to the high-pass characteristic of a hybrid, or two-wire four-wire conversion circuit. A typical waveform of such long-tail echoes is one that adopts a negative exponential curve. While the main portion of such a waveform can be cancelled, the remaining tail portion lies outside the range of tap weight control of the FIR filter and remains uncancelled, causing residual echoes to accumulate if .tau.&gt; N1, where it is the length of an echo, T is a symbol interval and N is the number of tap weights.
European Patent Application 0 281 101 (published on Jan. 31, 1990) discloses an echo canceller which is capable of cancelling long-tail echoes. However, it needs two recursive filters (Infinite Impulse Response filter) to form a tall canceller in addition to a nonrecursive filter.